1. Field of the Invention
This invention relates to telephone call dialling in a controlled credit environment and in particular but not exclusively to the use of telecommunications terminals in a prepaid environment where the terminals are capable of dialling telephone numbers and modifying the telephone number input by a user in order to obtain preferred call routing.
2. Description of the Related Art
Recent trends in telecommunications have resulted in a proliferation in the number of telephone networks and service providers available to a user of telecommunication devices such as telephones, facsimile machines and various types of computer based apparatus equipped with modems. A user wishing to initiate a telephone call for a communications session for audio, audio visual, facsimile, or digital data transmission with a call destination will generally input the telephone number of the call destination to the telecommunications device and will rely upon the network to which the device is locally connected to decide upon the route taken by the call between the local network and the call destination. Alternatively, the user may choose to access facilities of a service provider by adding a prefix to the call destination telephone number. The service provider may then provide services at reduced costs and may include routing the call in a manner determined by the service provider.
It is known from WO 00/07347 to provide a telephone which includes a route determining means which automatically modifies the number input by the user when dialling an outgoing call, the telephone having means for referring to an internally stored look-up table covering all possible call destinations to obtain routing information for determining the optimum route, thereby allowing an appropriate prefix code to be automatically selected and added.
Such arrangements however require the look-up table to be periodically refreshed with updating information in order to take account of changes in charging rates applied by service providers for given routes and to take account of variations in performance characteristics of various networks from which a selected route is to be chosen.
Co-pending United Kingdom patent application GB-A-2365259 describes an improved system in which routing data is obtained using an exchange of message between the telephone and a control centre wherein the telephone responds to the input of a dialled number by outputting a request message containing the dialled number to the control centre. A response message from the control centre contains routing data used by the telephone in placing a call to the required destination.
It is also known to provide a communications device operating in a prepaid service environment in which a subscriber prepays funds into an account of an operator and the cost of calls is debited from the account. The subscriber is required to maintain the account in credit for continued use of the service and for example cannot initiate a new call if remaining credit is zero or below a minimum threshold value and will have an existing call truncated if credit expires during the making of an existing call.
The control of the making of prepaid calls in dependence upon the account status of the subscriber is straightforward if the device is using a network controlled by the network operator with whom the subscriber has an account. If this is not the case however, as for example in the case-of mobile telephones which are capable of “roaming” operation in which the telephone registers with a network other than the subscriber's home network, it is more complex to control network access and billing in a prepaid environment. The present invention seeks to provide an improved method of routing telephone calls from a variety of telecommunications devices in a prepaid environment and a telecommunications device for use in such a method.